Preparation and characterization of colloidal αFeOOH with a narrow size distribution

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Abstract
Crystals of α-FeOOH with a narrow size distribution and maximum dimensions < 0.1 µm have been prepared by a two-step process. In the first stage, amorphous iron(III) hydroxide was converted to a crystalline, FeIIFeIII intermediate by interaction with a reducing organic ligand, L-cysteine. This intermediate phase was then oxidized to α-FeOOH. The initial crystals of α-FeOOH were poorly ordered. They transformed to a fully crystalline material by a dissolution/reprecipitation mechanism. The crystal morphology depended on the cysteine: Fe ratio, on the type of buffering agent used and on the presence of foreign cations in the system. α-FeOOH containing a few percent of Mn or Zn in the crystal structure could also be prepared by this method. Both the α-FeOOH and the crystalline intermediate phase were characterized using various techniques. α-FeOOH with a narrow size distribution has wide application as a model system for fundamental and technological studies.